1. Technical Field
This description pertains generally to electrical pickups, and more particularly to multiferroic pickups for electrical string instruments.
2. Background Discussion
Electric guitar pickup and dynamic microphone pickup technology has remained relatively unchanged since the solenoid/magnet style pickup was created in the 1930's. Small advancements in coil placement and magnet types have been made over the years (e.g., the humbucking pickup in 1955). FIG. 1 shows a diagram of a single string solenoid pickup 30 and the flux lines 38 impinging on a ferromagnetic string 32. As the string 32 is moved through the magnetic flux 38 generated by the polepiece (magnet 34), a time based current ε(t) is produced in the solenoid.
Magnetic induction based stringed instrument pickup devices operate on the principle of Faraday's Law of induction (Eq. 1) in that a time based change of magnetic flux through a solenoid will create a proportional electromotive force through the solenoid circuit:
                    ɛ        =                              -            N                    ⁢                                    δ              ⁢                                                          ⁢                              φ                B                                                    δ              ⁢                                                          ⁢              t                                                          Eq        .                                  ⁢        1            
A typical arrangement for this type of device comprises a large copper coil solenoid 36 surrounding a collection of cylindrical permanent magnets or biased ferromagnets 34. The magnetic pole pieces 34 rest directly underneath the strings 32, which are also constructed from ferromagnetic material. The pole pieces 34 serve to direct the magnetic flux path 38 toward the individual string being detected. When the string 32 is vibrated either by manual plucking or indirect striking, the high permeability of the string 32 acts to redirect the fringing magnetic flux lines, altering the magnitude of flux through the center of the solenoid and inducing a current. This current ε(t) is proportional to the strings velocity and reflects its fundamental resonant mode. The current is fed into a load and often times amplified for live performance or processed for musical recordings.
The magnetic pickups incorporating the technology of FIG. 1 have several drawbacks that limit their practical use. The primary issue in using a solenoid pickup is the large coil size relative to the strings displacement during operation. To achieve the output voltage necessary for amplification, a large coil must enclose all of the magnetic pole pieces and therefore will output a concatenation of each string vibration simultaneously. String spacing on many modern electric musical instruments does not allow the necessary coil geometry for proper isolation and decoupling of individual string signals. This prevents the equalization of naturally occurring frequency and amplitude variations under the instruments operating conditions. Introduction of non-fundamental resonant modes or excessive mechanical damping is also possible if the magnetic dipole coupling force is large enough between the pole piece and the ferromagnetic string. This results in a balance between sensitivity and the introduction of harmonic distortion when designing and aligning the pickups.
With the recent expansion in the study of piezoelectric and ferroelectric materials, several guitar manufacturers have successfully integrated piezoelectric pickups into their products. Because these types of pickups transduce the vibrations transferred from the string to the body of the instrument, the signal is often times corrupted by the resonant behavior of the guitar body material and geometry. This colored tonality is often best suited for hollow acoustic style instruments which have a mechanically resonating soundboard rather than the solid non-resonant body design of many purely electric instruments.